a) Field of the Invention
The invention is directed to an arrangement for the generation of intensive short-wavelength radiation based on a gas discharge plasma, preferably as a source of EUV radiation. The invention is applied in particular in high-power radiation sources for ELV lithography which requires radiation sources with electrodes having a long life in the process of industrial fabrication of semiconductor chips.
b) Description of the Related Art
In semiconductor technology, there is a continuing trend toward increasingly smaller structures, and radiation with increasingly shorter wavelengths is required for lithographic generation of these structures. At present, EUV radiation sources, viewed as the most promising lithographic tool, are being developed. Basically, there are two different ways of generating the radiating plasma: by laser (LPP) and by gas discharge (GDPP).
Various arrangements are known from the prior art relating to gas discharge-based EUV radiation sources, namely, Z-pinch, plasma focus, star pinch, hollow-cathode discharge arrangements, and capillary discharge arrangements. Further, there are variations in the above-named discharge types (e.g., hypocycloidal pinch discharge) and arrangements that combine elements of different discharge types. In all of these arrangements, a pulsed high-power discharge of >10 kA is ignited in a gas of determined density, and a very hot (kT>30 eV), dense plasma is formed locally as a result of the magnetic forces and dissipated power in the ionized gas.
However, the radiation sources must satisfy precisely defined requirements for use in EUV lithography under production conditions:
1. wavelength13.5 nm ± 1%2. radiation output in the intermediate focus115 W3. repetition frequency7-10 kHz4. Dose stability (averaged over 50 pulses)0.3%5. life of the collector optics6 months6. life of the electrode system6 months.
It is standard for high-power EUV gas discharge sources of the type mentioned above to have a special ceramic disk or cylinder as an insulator between the electrodes. For example, U.S. Pat. No. 6,414,438 B1 discloses a method and arrangement by which short-wavelength radiation is generated from a gas-discharge plasma in that a pre-ionization of the work gas takes place between coaxial electrodes as a sliding discharge on ceramic surfaces which emits UV radiation and fast electrons, and the ionized gas is conducted through an axial aperture of one of the electrodes in the gas discharge area, where it ignites the main discharge.
WO 03/087867 A1 discloses another high-energy photon source that generates EUV radiation in the range of 12-14 nm. In order to limit erosion of the electrodes, particularly of the center electrode and, therefore, to increase the lifetime of the electrodes, cylindrical insulators are arranged at the side walls of the center electrode so that the discharge current after pinch ignition is shifted over a larger area to another portion of the electrode. It is described as particularly advantageous that the center electrode is covered on the inner side and outer side with insulating tubing.
DE 101 51 080 C1 describes similar tubular insulator configurations that are also added to the inner wall of the outer electrode. Further, different materials are also indicated for this purpose. It is evident that while all of these insulator tubes limit the erosion of the electrodes to determined surface zones, the lifetime of the insulator/electrode configurations is appreciably shortened through cracking and metallization, particularly with high pulse repetition frequencies of the EUV gas discharge source.
For various reasons, the arrangements mentioned above always only meet the above-mentioned requirements (1-7) in a few respects. This can be explained using the example of star pinch discharge which, in itself, is advantageous. Because of the comparatively large distances between plasma and wall (which represents a severe problem in all of the other arrangements due to the otherwise small dimensions), the star pinch arrangement is characterized by a long electrode lifetime. However, the large dimensions of the star pinch discharges cause a luminous plasma with a length of more than 5 mm. This considerably reduces the efficiency of the collector optics and, therefore, the overall efficiency as a quotient of the output in the intermediate focus and the electric power introduced for the discharge. Electrode configurations which employ additional insulator tubes because of their short distances in order to improve the constant, stable generation of the plasma suffer in principle from premature failure of the insulator.